6.20.1 Introduction

Treating all possible electronic configurations within a wave function via full configuration interaction (FCI) provides the exact solution to the electronic Schrödinger equation. Exponential growth in the number of electronic configurations with system size makes this approach infeasible for all but the smallest systems. The method of increments, however, can be used to systematically capture electron correlation at polynomial cost while maintaining size extensivity. This approach, called incremental FCI (iFCI), utilizes a many-body expansion of the correlation energy, separating it into $n$-body terms. Higher-orders of $n$ converge toward the FCI solution and often do so without $n$ growing too large to be computationally burdensome. This method is highly parallelizable and features versatile truncation schemes.^{1297, 1298, 1299, 947}